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Optimum Slit Height in Photographic Sound-Track Reproducers
By W. K. GRIMWOOD and J. R. HORAK
For a specified reproducer frequency-response characteristic, there exists an optimum slit height. The optimum slit height depends upon the relative amounts of shot noise from the photosurface and thermal noise from the amplifier circuits. Calculated and measured values of optimum slit height are presented. The slit height which minimizes noise is undesirably large. Shot and thermal noise levels may be ignored if the d-c voltage drop across the effective phototube load resistor, without film in the light path, is of the order of 300 mv or higher.
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HE TERM "optimum" when applied to the height of the scanning slit in a photographic sound-track reproducer can be variously interpreted. One interpretation appearing in the literature of the subject is the slit height which gives maximum signal output at an assigned frequency,1 another is that slit height which results in maximum ratio of signalto-phototube noise at an assigned frequency.2 The definition of "optimum" taken as the basis for this paper is that value of slit height which gives maximum signal-to-system noise ratio for a specified frequency-response characteristic as measured from film modulation to amplifier output. Film noise plays no
Communication No. 1514 from the Kodak Research Laboratories, by W. K. Grimwood and J. R. Horak, Kodak Research Laboratories, Eastman Kodak Co., Rochester 4, N.Y., presented by J. R. Horak on October 10, 1952, at the Society's Convention at Washington, D.C.
part in the determination of optimum slit height. The effective size of the scanning beam is determined, not by the optical slit image, but by the overall response of the system. Since the reproducer frequency-response characteristic is fixed, the effective slit height is fixed. The reasonable assumptions are made that all phototube noise is shot noise and that all amplifier noise is thermal noise arising in the input coupling circuit. Both sources of noise have, therefore, the spectral distribution of "white noise" before any frequency discrimination is encountered.
The reproducer frequency-response on which are based the calculated and experimental data in this paper is one of the Standard Electrical Characteristics for Theater Sound Systems3 specified by the Motion Picture Research Council (Fig. 1). The same set of curves have been proposed as standards for 16mm review rooms.4
November 1952 Journal of the SMPTE Vol. 59
379